1. Field of the Invention
The present invention concerns a propeller of a pulsed airflow generator, in particular for a portable blower. It is of more particular interest that it is applicable to portable electric blowers. Such devices are commonly used to “sweep” dry leaves or various debris from various surfaces such as lawns, city streets, outside parking areas, sports grounds, play grounds, etc.
The invention also applies to portable devices, in particular to self-powered blowers featuring a pulsed air generator making use of this method and this propeller.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Axial fans used in the manufacture of blowing machines comprise a propeller consisting of a number of radial, straight or twisted blades that are attached at their base to a central hub.
Blowing machines using propellers with straight blades have the disadvantage of weak performance, so that it is often preferred to use propellers with twisted blades which perform better.
In fact, the profile of each blade acts somewhat like the wings on an aircraft. It presents, at the air inlet, a leading edge and, at the air outlet, a trailing edge. For optimal performance the air must arrive at the leading edge in an essentially perpendicular manner. On the other hand, in order to achieve optimal operation of the fan, in application of the principle of the velocity triangle (FIGS. 8, 8A, 8B), one seeks to create, through the rotation of the fan, an airflow at speed Va that is as uniform as possible over the entire surface of the fan and thus to obtain a speed Va. Now, if we consider the profile of a blade root, it rotates around the axis of the fan, essentially at a speed VΩ equal to the speed of rotation multiplied by the radius of the blade root where this profile is positioned. This profile thus receives the air at a relative speed Vr. If one considers the profile positioned at the end of the blade, it also generates an air speed Va through the rotation of the propeller, but it turns at the same rotational speed multiplied by the radius of its position relative to the axis of the fan which is considerably greater than the radius at the blade root. This results in a relative airspeed Vr that is much greater but has an orientation that is further away from the axis of the fan than at the blade root. This principle explains the fact that a high-performing propeller should be constituted by twisted blades which does not make its manufacture any easier.
Therefore, one tends to position the beginning of the blades as far away as possible from the axis of rotation in order to reduce the differential of orientation of the speed Vr between the blade root and the blade top, thereby simplifying the manufacture of the blades somewhat. But on that account it becomes necessary to integrate in front of the propeller a cone for improving the aerodynamics of the air inlet. It is clear that the length of this air inlet cone is proportional to the radius of the blade root, the length of said cone increasing with the radius of said blade root. The fans needing this type of cone have therefore greater space requirements which is a definite disadvantage for those applications where high performance is sought for portable devices with a pulsed air generator of reduced volume.
When blowers of large dimensions need to be built where the blades are fastened on the central hub, either through welding or through mechanical fasteners, their construction does not create big problems.
On the other hand, when one wants to apply such propellers to the production of axial fans of smaller size, such as those in axial fans producing the pulsed airflow in sweeper and/or cleaning blowers for which this invention is more specifically intended, molding such small-size propellers becomes practically impossible, unless injection equipment or methods are applied which require the use of very complex molds (loose-detailed molds). This results in higher costs for tooling and, consequently, of the manufactured products, and also leads to fragility of the mold and thus a reduction of its useful life which means an additional increase in the cost of the parts. Most of the time, a propeller of this type cannot be molded with existing methods and equipment, and the designer is therefore inclined to make a compromise on the geometry of the propeller so that it can be produced by molding.
Besides, the parts produced by such equipment and methods may lack precision and show major variations from one lot to the next.
It has been thought to remedy these disadvantages by proposing axial propellers where the blades are attached to a cylindrical hub or with blades with reduced twist, or with a reduced number of blades, to the detriment of the performance of the propellers produced this way.
Document US-2010/007476.1 describes a pusher-type airscrew constituted by the assembly of two parts coaxially attached one after another, like this: a first front part consisting of a single block and constituted by a number of twisted blades attached to a central ring, a second rear part, of a general conical shape also consisting of a single block. The two parts are assembled by engaging the root of the blades of the propeller into slots provided on the second rear part. The central part of this assembly presents the shape of a truncated, cone, the vertex of which consists of a flat, relatively large area which interferes with the aerodynamics of the air inlet.
Document U.S. Pat. No. 4,462,757 discloses a propeller of a cooling fan also consisting of two parts that are coaxially attached one after another, like this: a first front part formed by a single block and consisting of a number of twisted blades attached to a central ring, a second rear part of a general conical shape, also formed by a single block. The two parts are assembled with screws. The central part of this assembly presents the shape of a truncated cone the vertex of which also consists of a flat, relatively large area disturbing the aerodynamics of the air inlet.
Document DE-28 55 909 also describes a propeller for a cooling fan of an internal combustion engine, constituted by the assembly of two parts coaxially attached one after another, like this: a first front part formed of a single block and constituted by a number of twisted blades attached to a central ring, a second rear part of a general conical shape also consisting of a single block. According to this document, the first and second parts are also assembled by a number of screws. The central part of this assembly presents the shape of a truncated cone the vertex of which is also constituted by a flat, relatively large area disturbing the aerodynamics of the air inlet.
One aim of the present invention is to propose a propeller without the afore-mentioned disadvantages.